1. Field of the Invention
The invention relates to a braking force control apparatus and a braking force control method that perform a front-rear braking force distribution control that distributes a braking force to front wheels and rear wheels.
2. Description of the Related Art
A braking force control apparatus that performs a front-rear braking force distribution control that distributes a braking force to front wheels and rear wheels has been known. For example, Japanese Patent Application Publication No. 10-138895 (JP-A-10-138895) describes a braking force control apparatus that controls the braking forces applied to the left and right rear wheels individually so that a difference between the greater one of the wheel speeds of the left and right front wheels and the wheel speed of the left and right rear wheels reaches a value in a predetermined value.
In a vehicle equipped with a conventional braking force control apparatus that performs the front-rear braking force distribution control, similarly to the braking force control apparatus described in Japanese Patent Application Publication No. 10-138895 (JP-A-10-138895), the vehicle sometimes wobbles, if braking is performed during a high-speed travel.
Generally, the center of gravity of a vehicle is shifted from the center of the vehicle in the left-right direction. It is considered that the foregoing phenomenon occurs because the left and right wheels have different ground contact loads. In the case where a braking force is applied to left and right wheels, the wheel whose ground contact load is smaller achieves a greater effect of deceleration. Accordingly, the vehicle tends to turn toward the side of the wheel whose ground contact load is smaller, even if the steered wheels are in the straight travel position. Thus, an unintended yaw rate occurs in the vehicle, and the vehicle subjected to an unintended roll moment to roll because of the unintended roll moment. As a result, the vehicle wobbles left and right.
As the vehicle rolls, shifting of load in the transverse direction of the vehicle occurs, and therefore the ground contact loads of the left and right wheels fluctuate. When the braking force is controlled so that the braking slips (the degrees of slip at the time of braking) of the left and right rear wheels relative to the braking slip of the front wheels become equal, according to the front-rear braking force distribution control, the braking forces applied to the left and right wheels are controlled so as to be proportional to the ground contact loads thereof.
However, the roll behavior of a vehicle is in a first-order lag relation with the unintended yaw rate of the vehicle. Therefore, although the braking forces applied to the left and right rear wheels are controlled according to the front-rear braking force distribution control, the wobble of the vehicle caused by the unintended roll moment may not be restrained and the roll behavior of the vehicle may deteriorate, because of the control lag.